Analysis on the printing process of the hottest mi

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Brief analysis on the printing process of milk powder packaging bags (II)

printing process and control

vacuum aluminized composite packaging materials mainly include BOPP/PE/VMPET/PF, BOPP/VMPET/PE and PET/VMPET/PE

1. process flow

bopp film or PET film is printed, compounded with polyester aluminized film through dry compounding process or extrusion compounding process, compounded with polyethylene heat sealing layer through dry compounding process, and finally the finished product is obtained through slitting and bag making. The milk powder packaging bag belongs to the terminal packaging of milk powder, so its printing quality and composite quality are regularly coated with a thin layer of molybdenum disulfide grease; Very high requirements, so generally should choose high-quality composite printing ink and dry composite adhesive

2. ink working viscosity control

the viscosity of ink directly affects the transfer rate, levelness, scraper suitability and other properties of ink, and is the most important indicator of ink performance. In the printing process, by adjusting and controlling the ink viscosity, the ink can meet the needs of the printing process and achieve the color consistency and balance in the printing process. The change of ink viscosity will affect the color concentration and transfer performance of ink. Drying speed, etc. When the viscosity is high, the color concentration is relatively high, on the contrary, the color concentration is also relatively low; Viscosity change can also directly affect the transfer rate of ink, and then affect the color of printing products, especially for products with lighter color. Therefore, in gravure production, the viscosity of the ink must be strictly controlled. Generally speaking, the working viscosity of the ink should be controlled within 15 ~ 26 seconds (3n cup), and can be controlled according to the printing speed. The printing plate depth and color shall be adjusted

in high-speed printing, the ink separation speed is fast, and the ink needs to work at a low viscosity, otherwise the ink transfer effect will be affected. When the viscosity of some low-grade inks drops to a certain degree, there will be faults such as water marks, so low-grade inks can not meet the requirements of high-speed printing. A printing plate with deep lines, such as a line plate. Field version requires high ink viscosity; However, for shallow layers, especially those with more highlights, the viscosity should be lower. In addition, we should try to use ink with good performance and wide viscosity range

for the printing ink, the ink viscosity is determined by the proportion of solvent added, the solubility of solvent to ink, ink temperature and solvent balance

(1) proportion of solvent (relative amount)

"in actual production, solvent is mainly added to reduce ink viscosity. Gravure ink solvents generally include alcohols, benzene and esters. Ketone solvents, etc. If the ink can be diluted in any proportion, the viscosity will decrease regularly with the increase of solvent content. But in fact, some inks add more solvents and the viscosity drops slowly. Some inks add a small amount of solvents and the viscosity drops sharply. Too much solvent will cause the imprinting edge to be not smooth. When the dilution exceeds a certain proportion, the ink viscosity will tend to a fixed value, and it is easy to cause white spots. Printing faults such as water marks and poor gloss

when diluting the original ink with solvent, the solvent must be slowly poured in while stirring. Rapidly diluting the ink with a large amount of solvent will cause "solvent impact", which will cause imbalance in the proportion of local solvents in the ink, and the pigments evenly distributed in the resin will be exposed and lose luster

(2) solvent solubility in ink

the added solvent has good solubility in ink, and the viscosity of ink is low. Therefore, under the same solvent ratio, the ink viscosity of different solvent combinations is also different. Generally, solvents are not random signal inspection, harmonic distortion and other accuracy indicators. They are formulated to ensure the uniqueness of environmental experimental conditions. The verification items are used only, taking into account the solubility and drying speed. Economic and environmental requirements, use mixed solvents

the solubility of mixed solvents is not a weighted average of the solubility of a single solvent. Generally, the solubility of mixed solvents is stronger than that of a single solvent. For specific inks, specific solvents and proportions should be used. If the solvent is misused, the ink will solidify, precipitate and change color in serious cases, which can be found in time; When the impact is slight, the ink seems to have dissolved, but it will change color and block the layout during the printing process. Water marks and other phenomena

(3) ambient temperature and ink temperature

due to the circulation of ink and the heat generated by the friction of printing machinery, the ink temperature will gradually change and cause the fluctuation of ink viscosity. The test shows that the temperature change of 1 ℃ will lead to the viscosity change of 3%. Therefore, in winter, the ink should not be placed outdoors. If gelation occurs, it should be preheated in advance to reduce the temperature change of the ink itself. Ink temperature not only affects the viscosity of the ink, but also affects the drying speed, transfer rate and point reduction. Printing gloss and so on

(4) volatilization of the ink solvent in the ink tank

as the volatilization of the solvent will affect the working viscosity of the ink, it is necessary to supplement the solvent in time. Gravure printing ink uses solvent with strong solubility and fast volatilization. With the progress of printing, the solvent in the ink volatilizes continuously, which will lead to the increase of ink viscosity. For this reason, we should pay attention to the change of viscosity at any time during printing. According to the loss of solvent, we should use viscosity controller or manual timely supplement solvent to maintain the stability of viscosity

(5) solvent imbalance

due to different solvent volatilities, some solvents will volatilize during the printing process. If the real solvent volatilizes first, the solvent proportion in the ink will be damaged, and thickening and even resin precipitation will occur. Therefore, in the printing process, if the ink consumption of printing pictures and texts is small, the solvent should be added in time; When the solvent balance of the ink is poor, some solvents with slower volatilization speed should be added

3. ink dryness control

the influence of ink drying speed on the tone level and residual solvent of printed products. Odor has a decisive influence. The purpose of adjusting ink drying performance in the production process is to control the drying speed of ink

gravure printing ink is a volatile dry ink. When printing, adjusting the drying performance of the ink according to the specific printing and drying conditions (drying time, drying air temperature, drying air speed and environmental conditions, etc.) is an important condition to ensure the smooth printing

(1) drying speed of ink

the general principle for selecting the drying speed of ink is that the ink can be well transferred from the gravure cavity and has good solvent balance and leveling after transfer; After the ink transfer forms the ink film, the solvent in the ink layer escapes from the system in an instant, making the printed image and text dry rapidly. The following problems may be caused when the ink drying speed is too fast:

① at the moment when the ink in the gravure cavity reaches the substrate, the transfer rate of the ink is reduced due to the rapid volatilization of the solvent and the increase of the viscosity, which affects the color of the print, resulting in insufficient ink in the high light part and poor ink dispersion in the dark part. Some small workshops produce unqualified plastic bags and printing failures such as dry plate

② a solvent volatilizes too fast to make the system out of balance, resulting in resin insolubility. White turbidity, local resin whitening such as gel. At the same time, due to the rapid decrease of the ink layer temperature, the water in the air becomes water droplets and dissolves into the ink, causing "water whitening" of the gel, resulting in poor gloss of the printing ink layer

③ excessive drying of the ink increases the friction between the printing plate and the scraper, which makes it difficult for the scraper to clean the printing plate ink, resulting in printing faults such as knife line

when the drying speed of the ink is too slow, the residual solvent may increase, resulting in the adhesion of the printing product and the appearance of peculiar smell in the packaging. A small amount of ink in the non graphic part is transferred to the substrate to form dirt, which will have an adverse impact on the post process such as compounding

(2) factors affecting ink dryness

① initial properties of ink. The initial dryness of ink is affected by the release of solvent and resin, the particle size of pigment and other factors. In fact, the ink suitable for different printing speeds and plate conditions is different in the design of initial dryness

② volatilization rate of mixed solvent. Solvent evaporation drying mechanism of gravure printing ink. The evaporation rate of the solvent directly determines the drying performance of the ink. Table 1 lists the relative volatilization rates of commonly used solvents

the volatilization rate of solvent in gravure printing ink is not only related to the type of solvent used, but also related to the release of resin and pigment to solvent. Generally, the volatilization rate of mixed solvent in the ink is lower than that of pure solvent. Therefore, gravure printing inks often use mixed solvents to adjust the most appropriate solvent volatilization rate through different types and different proportions of solvent combinations

as gravure printing ink uses mixed solvents, when the printing ink forms a film on the substrate, the highly volatile solvent first escapes, and the less volatile components stay behind, which changes the components of the solvent, and the volatilization of the solvent gradually slows down. In addition, during the process of ink changing from liquid to solid, the viscosity increases continuously, and the volatilization speed slows down. If the combination of solvents does not meet the requirements, such as too many high boiling point components or too many slow drying solvents, serious residual solvent problems will occur under normal drying conditions

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